Mobility of the Negative Ion. 



453 



critical value Xi of the electric force, at which the mobility 

 of the negative ion becomes abnormal, is very small in 

 hydrogen and increases with the amount of impurities to 

 such an extent that it may serve as a good criterion for 

 testing the purity of hydrogen. 



The curves I., II., and III. (fig. 6) represent the mobility 



Electric force in gg* 



of the negative ion at pressures 750, 375, and 100 mm. 

 respectively, in the purest hydrogen obtained for these 

 experiments. The critical value X x of the electric force is 



about 40 - - at atmospheric pressure, and at lower pressures 



cm. 



it becomes too small to be measured by this method. The 



value of — corresponding to the maximum point of the 



mobility curves is very large, varying from 1*8 at atmo- 

 spheric pressure to 1*1 at 100 mm., and with further increase 

 of the electric force the mobility tends, as in air, to attain a 



minimum value. These high values of — are probably due 



to the impurities contained in the hydrogen. Experiments 



